In conventional charged-particle-beam (CPB) pattern-transfer methods, an electron beam (or other charged-particle beam) is used to transfer a pattern from a mask to a wafer in order to obtain the high resolution imagery characteristic of CPB optical systems. Although charged-particle beams provide high-resolution imaging, pattern transfer using CPB pattern-transfer methods is generally slower than pattern transfer using optical radiation. Methods and apparatus have been disclosed that attempt to speed pattern transfer and increase throughput using CPB methods.
One CPB pattern-transfer method is known variously as cell-projection, character-projection, or block exposure. This method is suitable for transferring repetitive circuit patterns defined by a small area of the mask to a wafer or other substrate. In this method, several so-called "unit patterns" are defined on the mask and these unit patterns are repeatedly transferred onto the wafer using a charged-particle beam. For this method to be useful, the unit patterns should be no larger than about 5 .mu.m by 5 .mu.m as projected onto the wafer. In addition, the patterns to be transferred must consist primarily of repeats of the unit patterns and the method is unsuitable for patterns that are not repetitive. Using this method in the production of semiconductor integrated circuits such as DRAMs, throughput can be as much as ten times larger than using methods that do not take advantage of pattern repetitiveness.
Another method for increasing throughput in CPB pattern transfer is a demagnifying pattern-transfer method disclosed in laid-open Japanese Patent Document No. Hei 5-160012. In this technique, a mask is used to define the patterns for an integrated circuit. The mask is divided into multiple of fields that are further divided into a plurality of subfields. An electron beam illuminates a subfield and a demagnified image of the subfield is projected onto the wafer with a two-stage projection lens system. The images of the patterns in the remaining subfields are successively projected onto the wafer so that the projected subfield patterns are aligned to form the patterns for the integrated circuit. Because the electron beam does not illuminate the entire mask simultaneously, alignment of the projected subfields is an important consideration. To achieve accurate subfield placement, the CPB optical system can be adjusted to improve the placement and resolution of each subfield image. This method is referred to as a "divisional" pattern-transfer method and is disclosed in, for example, U.S. Pat. No. 5,260,151. This method has not been successfully demonstrated for commercial production of integrated circuits.
The CPB pattern-transfer methods described above are useful only if the patterns in the subfields are accurately stitched together on the wafer. Laid-open Japanese Patent Application No. Sho 63-1032 discloses a method for improving the accuracy with which patterns are stitched together. In this method, a common pattern is formed in edge portions of two subfields whose pattern images are to be adjacent on the wafer. The subfields are projected onto the wafer so that the images of the common pattern in the two subfields overlap. The common pattern formed in the edge portions is exposed twice at a relatively low dose in different exposures so that the total dose (i.e., the total exposure to the electron beam) in the edge portions is approximately the same as that of other areas.
Unfortunately, the JP 63-1032 method is only useful in pattern transfer using a variably shaped beam or a focused beam, and no effective method for improving the accuracy of the pattern stitching, which is generally applicable to a mask-pattern transfer process using an electron beam, has been established yet. In particular, a method for efficient control of the dose distribution within a unit exposure area has not been proposed yet.
In shaped-beam pattern-transfer methods, the charged-particle beam is generally shaped with a shaping aperture. The boundary of a subfield transferred to a wafer is defined by the image of the shaping aperture on the mask instead of an edge or other feature on the mask. Distortion or blur of the shaping aperture image degrades the quality of the transferred patterns. The illuminating charged-particle beam scans the entire field. If the field is large, then the image of the shaping aperture exhibits distortion, particularly at the most off-axis portions of the field. If the distortion is large, then the total exposure varies in the double-exposure area on and near the pattern-connection boundary. This can cause inaccurate stitching of the patterns in adjacent fields.
It will be apparent from the foregoing that improved methods and apparatus are needed for CPB pattern transfer.